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Abstract It is well known that the Multiple-Input Multiple-Output (MIMO) systems can significantly improve the capacity and reliability of communication over fading channels using spatial multiplexing and/or space-time coding. However, it is difficult to equip the small mobile unit with more than one antenna with uncorrelated fading. In such case, the transmit diversity can only be achieved through what is known as cooperative diversity, where the users in the network share their resources and transmit cooperatively. First, we propose a cooperative diversity protocol implementing space-time coding for an arbitrary number of relay nodes with Amplify-and-Forward (AF) protocol, when the sourcedestination link contributes in the second phase. We consider the use of real-orthogonal and quasi-orthogonal designs of space-time codes as they give better performance than random linear-dispersion codes. It is proved that the proposed protocol achieves a diversity of order 1 N + , where N is the number of relay nodes. The optimum power allocation that minimizes the PEP is derived using numerical and theoretical analysis. Then, the design of a distributed Space-Frequency Code (SFC) for multipath fading relay channels that can exploit both cooperative and multipath diversities is considered. It is shown that the proposed code achieves a diversity of orderNL , where L is the channel memory length. The Decode-and-Forward (DF) protocol and erroneous decoding at the relay nodes are considered. We also consider the design of cooperative diversity schemes for the Single-Carrier Frequency Division Multiple Access (SC-FDMA) system in the uplink direction. Specifically, two relay-assisted distributed space-time/frequency codes are proposed. The proposed distributed SFC takes into account the Peak-to-Average Power Ratio (PAPR) which is a major feature of the system. Moreover, we propose a bandwidth-efficient distributed Space- Time Code (STC) for slow fading relay channels. DF protocol is used at the relay nodes. Finally, we investigate the impact of cooperative communication on improving the physical layer security. The proposed techniques select a trusted DF relay to assist the source transmission and maximize the achievable secure rate for different eavesDROPper policies (cooperation or not) and available channel knowledge. In addition, a jamming method that selects two relay nodes to create an intentional interference at both phases of the DF protocol in order to confound the eavesDROPper nodes is also considered. Simulation and analytical results demonstrate the performance improvement of the proposed schemes. |